It is quite common for persons refueling their vehicles to add additional fuel after the initial fill nozzle shut-off. Often this is done in an attempt to "round up" the payment to some convenient amount to minimize the amount of "change" involved in the transaction. Surveys have indicated that providing some amount of "round-up" during refueling is a desirable customer feature. A less frequent occurrence is for the operator to attempt to maximize the amount of fuel put into the fuel tank. This is typically accomplished by dispensing the fuel slowly over an extended period of time. This is generally described as "trickle-fill" by those involved in fuel system development.
The amount of "round-up" or "trickle-fill" needs to be limited so as to prevent overfilling of the fuel tank. The fuel tank must retain sufficient vapor space above the fuel to allow vapors to vent to the canister under such conditions as thermal expansion and parking on grades.
A control valve made by Stant Manufacturing Inc., Connersville, Indiana serves to vent fuel vapor from the vapor space in a fuel tank during early stages of refueling. In addition, the control valve blocks introduction of fuel in excess of a nominal volume so as to preserve a volume of the vapor space in the fuel tank once the fuel tank is filled to its rated capacity. The control valve is one part of a comprehensive Onboard Refueling Vapor Recovery (ORVR) vehicle fuel system that allows pump operators some latitude to "round-up" their fuel tanks without overfilling the fuel tanks. During the "round-up" phase of filling the fuel tank, a single bleed orifice provided in the control valve allows the valve to open to provide venting of the fuel tank.
One problem with having a single orifice in the Stant control valve is that it brings into conflict two system requirements. The first requirement, the ability to add fuel ("round-up") after the initial nozzle shut-off, requires a larger orifice to allow the valve to reopen quickly so more fuel can be added to the fuel tank. The other requirement, to discourage the pump operator from continuing to "trickle fill" so as to prevent overfilling the fuel tank, requires a smaller orifice to delay reopening of the valve. In order to fulfill both requirements, the single orifice arrangement results in a compromise orifice size which is not entirely satisfactory.